The path of a ball in flight is determined, in great part, by the amount of spin and the direction of spin imparted to the ball upon impact. For example, the path of a golf ball is determined by the launch conditions measured over the first few feet of flight after the golf ball is struck by a golf club. The parameters that are used to determine the path of a golf ball are spin, launch elevation, launch azimuth and speed. Knowing the precise values of ball spin provides the golfer with important feedback for modifying his or her swing to obtain desired golf game results.
Continuing with the golf ball example, devices that provide immediate feedback of the initial spin conditions of a golf ball may be utilized by the golfer and the golf shop professional for training purposes as well as for fitting a set of golf clubs to the golfer. Such devices may also be used to test the effectiveness of golf club and golf ball design modifications by measuring the effects of the design modifications on the golf ball launch parameters. By analyzing these effects, design modifications can be evaluated and improved. Other applications of these devices include golf simulators which project the flight of the ball in a video presentation based on initial golf ball launch conditions. The aforementioned prior art devices require complex camera systems for calibration/alignment, spin measurement, triggering, image capture, and trajectory measurement which are too expensive and difficult to use for the typical golfer and golf shop professional.
Calibration and Alignment. Accurate measurement of ball flight parameters requires knowledge of the optical characteristics of a camera that is used to track the ball. Prior art systems use complex targets in the field or other optical devices to calibrate cameras. These devices are cumbersome, fragile and also must be calibrated at regular intervals. Typically, positioning the target into the field of view of a camera or cameras and obtaining the correct lighting conditions and proper focusing requires skills that are not user friendly and exceed the capabilities of the average golfer, user, and/or professional.
There are various methods for field-calibrating the camera or cameras that are commonly used in prior art systems. A first method utilizes a target with dots which is set up where the ball will be struck. The camera finds the center of each of the dots to determine a frame of reference. The target is removed and the player hits a shot. The camera then uses the frame of reference to determine how the ball is moving. This method gives very crude measurement results since the number of calibration points are too far apart to properly account for lens distortion, and the target is placed manually so that precision location of the dots cannot be determined.
A more sophisticated method is to have a target with precise reference markers, known as “fiducials”, such as squares or circles, with known geometry. Software of the prior art system determines the edges or centers of each of the marks, and creates a mathematical model of the camera and lens, taking into account the distortion caused by the camera and lens system. The target is imaged at two known positions to provide a set of calibration points in three dimensions.
Prior art systems also require special precision optical alignment targets procedures that are necessary in order to locate a reference plane in space which is used for all azimuth and elevation measurements. The precision alignment targets must be imaged by the system cameras prior to each system setup. These complex alignment procedures, which require bulky and easily lost alignment targets, are difficult for lay personnel to learn, to set up and to move from place to place. In addition, the prior art alignment procedures usually require special lighting and focusing procedures for the alignment targets. Thus, the need for specialized alignment equipment, with the inherent costs and bulk, render the prior art systems inaccessible for general use, e.g., use by the public.
Spin Measurement. There are a variety of prior art systems that measure the initial spin conditions of a golf ball. A typical system uses strategically placed retroreflective or non-retroreflective dots, equatorial striping, or other specific marks on the ball. The ball then must be aligned to face the camera of the imaging system. The markings must be applied with precision, and are subject to obliteration or loss with repeated usage of the marked golf ball. The position of the dots/markings relative to a known position on the ball or relative to other dots/markings plays a central role in the precision of the measurement of the golf ball flight parameter measurements. Thus, applying these markings to the golf ball may require the use of special, costly equipment. In addition, specially marked balls preclude the ad hoc usage of a golfer's own golf balls, and require purchase of the marked balls from specialized manufacturers.
Triggering. Prior art systems for flight parameter measurement also require a separate triggering mechanisms for determining when the camera(s) should commence recording sequences of a golf ball or golf club movement, and when the measurement equipment should begin measurements. The triggering mechanisms typically involve acoustic or photo detectors which detect the moment of impact of the golf ball by the golf club. These mechanisms have significant limitations. The acoustic triggering device uses a microphone to detect the sound of the impact of a golf club on a ball. Other triggering devices employed by the prior art systems are based upon optical sensor technology for detecting an event, for example, when a golf club swings through an optical plane, e.g., a laser.
In addition to the disadvantage of requiring an additional piece of equipment for triggering, the prior art triggering devices are problematic and present disadvantages that render these devices impractical for the average golfer. For example, acoustic pickups often miss the trigger event or misfire upon receiving extraneous sounds. Trigger sensors often will not function properly when a golf ball is struck from natural, living turf. Remote photo/optical trigger sensors often require alignment and special overhead lighting since photo triggering devices typically will not function properly in an outdoor or sunlight environment. Also, laser-based sensors have a fixed, limited field of view. Trigger sensors which are remote from the measurement apparatus are easily lost or damaged, and require mounting in special ground level assemblies. In addition, precise golf ball placement is required for photo sensor or laser based triggering devices. A further disadvantage of both acoustic and optical sensors is that these sensors must be calibrated, and thus, require yet additional calibration devices.
Image Capture. Prior art cameras use the technique of multiple exposures to track the flight of the ball. The lenses are opened, and a flash strobe is used to create multiple exposures of the ball. However, this technique renders accurate analysis of ball motion difficult since multiple images are superimposed on top of each other. The multiple exposure technique will also capture multiple images of stationary background objects in the field of view that build up in intensity or bloom, potentially obscuring the images of the ball in flight. As a result, the camera must be placed in a location where the background is neutral to create the required contrast with the ball when illuminated.
Trajectory Measurement with a Single Camera. Prior art single camera systems measure spin and launch angle in a plane that is orthogonal to the field of view (FOV) of the camera. Speed, spin and launch angle are calculated in two dimensions introducing significant errors into the measured flight characteristics. Additionally, such a system cannot measure azimuth.
Prior Art Example. Prior art measurement systems as discussed above may be single or multiple camera systems. FIG. 3 illustrates a typical dual-camera measurement system 100 of the prior art. The measurement system 100 uses two area cameras 106, 108 that are positioned at two angles to a target position 102, such as a golf ball and tee. Typically, two types of sensor technologies, charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS), are used in the cameras 106, 108. CMOS sensor technology allows some processing electronics to be included on the same chip as the sensor within the camera 106,108. High bandwidth camera-specific data cables 122 are required to transfer data from the area cameras 106, 108 to a vision processor 112. A typical high bandwidth data stream transfer is forty (40) million pixels per seconds, i.e., 500 Mbits per second for pixels of eight (8) bits. The mega-pixel data stream is transferred over the camera-specific cables 122 to frame grabber modules 114 in the vision processor 112. Frame grabber modules 114 utilize standard integrated circuit (IC) boards to digitize an analog video stream image from the cameras 106, 108.
Continuing with FIG. 3, the digitized images, represented by arrays of numbers, are streamed to pipeline vision processors 116 for preprocessing. The pipeline vision processors 116 utilize dedicated image processing boards for data and image analysis. For example, a pipeline vision processor 116 may be configured to extract specific information from an image. The processed images from each of the pipeline vision processors 116 are sent to an image analyzer processor 118 that further analyzes and processes multiple images of a golf ball 102. The golf ball parameter measuring system 100 of the prior art may further include an image buffer board 120 for data storage. The vision processor 112 of the prior art requires a chassis to house the IC boards of the frame grabber module 114, the pipeline vision processor 116, the image analyzer processor 118, and the image buffer 120. The processed image from the vision processor 116 is sent to a host computer 104 for display on the graphical user interface (GUI) of the host computer 14. The host computer 104 may also include a database for the storage of golf ball characteristics which can be correlated with a particular golf club which was utilized to launch the golf ball.
The golf ball flight parameter measuring systems 100 of the prior art present several disadvantages. As described above, prior art golf ball flight parameter measuring systems 100 require special markings on the golf balls. At least two cameras 106, 108 must be used to achieve precision flight parameters, as well as special optical alignment equipment and triggering equipment 122 to align the cameras and initiate the measurement system after a ball is struck. Thus, there exists a need for a device and method for measuring the in-flight characteristics of a golf ball which provides precision measurements utilizing unmodified golf balls, a single camera for easy set-up and operation, and which does not require cumbersome and error-prone alignment and triggering equipment.